The present invention relates to a transparent vacuum insulation plate of a type having various uses, such as in the construction of a refrigerated display case, as a peep window on a combustion furnace, and as an insulated container in which high temperature contents are kept.
An insulated face of a refrigerated display case should be transparent so that the contents placed therein may be seen from the outside. For this reason, insulation materials such as foamed styrene or foamed urethane are not suitable. Hence, for many display cases, dried air is sealed in a space between double-glazed insulated faces in order that the inside of the case can be seen from its outside. In such display cases, heat conduction is reduced because of the poor heat conduction characteristics of the dried air. Since the sealed air is dry, there is no dew condensation on the inside of the glass of a low temperature face to affect transparency. However, despite the advantages of the sealed air-type display cases described above, heat transfer by convection in dried air precludes these type display cases from being perfect insulators.
A state of combustion can be observed through a peep window located on the wall of a combustion furnace or the like. However, a peep window does no intercept heat from the inside of the furnace. Therefore, whenever an observer wishes to open the peep window, he must put on heat insulated glasses or wear a heat insulated coat. Naturally, this causes an inconvenience.
The above problems are all solved by employing a transparent insulation plate with excellent insulation characteristics. A vacuum insulation plate is the best structure for both transparency and insulation characteristics.
In the above regard, technology wherein a high or low temperature is maintained within an evacuated double-wall has been disclosed in Japanese laid-open patent application No. 1979-63453.
The present invention concerns an evacuated double-wall insulation structure of a type wherein a honeycomb-type spacer is inserted to enable the structure to withstand a negative pressure. Insulation structures of this type minimize the heat transfer caused by convection. However, some heat is transferred through the spacer. The heat conduction of a spacer may be reduced by increasing the heat transfer resistance. In other words, the cross-sectional area of the spacer must be reduced, while the path of heat conduction therethrough must be extended.
Thus, of the three modes of heat transfer--conduction, convection and radiation--convection is intercepted and conduction is also reduced considerably. However, when installing a transparent insulation structure, heat transfer by radiation still remains a problem.
Heat radiation was not a problem with conventional insulation structures formed with opaque insulation materials (such as styrene or urethane) or with opaque outer walls. In this respect, the radiation was completely intercepted by the opaque metallic outer walls.
Both heat radiation and light transmittance are phenomena of electromagnetic wave transmission. Hence, a radiation problem exists in making a transparent insulation plate. As used herein, "transparency" means transparency against visible light. A wave length of a heat ray, on the other hand, is longer than that of visible light. As used herein, "visible light" has a wave length of 0.4.about.0.7 microns; a "heat ray" is an infrared ray which has a wave length ranging from 0.7 microns up to 1 mm. Thus, in order to solve the problem with heat radiation, something has to be done so that infrared rays are intercepted, while visible light is transmitted.
It is well known that metallic films have selective transmittance characteristics. Since metal is a good conductor, an electric current occurs in direct proportion to an electric field. However, a metal possesses a finite dielectric constant .epsilon.. A displacement current is given by time differentiation of the dielectric flux density D. Total electric current, therefore, comprises the electric current occuring in proportion to the electric field (field current) and the displacement current.
The field current generates Joule heat, thus consuming energy. The field current is also in direct proportion to conductivity .sigma., while the displacement current is in direct proportion to .iota..omega..epsilon.. As used herein, an imaginary unit is represented by i, 2.pi. times the frequency (angular frequency) by .omega., and the dielectric constant by .epsilon..
The higher the frequency, the less the ohmic electric current. That is, with higher frequency an electric current generating Joule heat decreases, and a displacement current increases. In other words, electromagnetic waves with high frequencies have less attenuation in a metal.
So-called semi-transparent films, which intercept most of the infrared rays and transmit a considerable volume of visible light, are made by pre-selecting a thickness d of a metal. The conductivity .sigma. and the dielectric constant .epsilon. are governed by the type of metal selected. It is desirable that an angular frequency of an electromagnetic wave, which should be a boundary, approximate .sigma./.epsilon..
For a semi-transparent film, various kinds of metals such as aluminum, copper alloy, zinc, etc. can be used. Semi-transparent sheets made by vacuum-metallizing transparent sheets (made from plastic using polyethylene, etc.) are now available on the market. These sheets are often placed on window glasses because they intercept heat rays.
In view of the above, an object of the present invention is to obtain a transparent vacuum insulation plate.
An advantage of the present invention is the provision of a transparent vacuum insulation plate which prevents nearly all heat transfer by convection and conduction.
Another advantage of the present invention is the provision of a transparent vacuum insulation plate which prevents heat transfer by radiation as well as by convection and conduction.
Yet another advantage of the present invention is the provision of a transparent vacuum insulation plate formed in the shape of a square container.